1. Field of the Invention
This invention relates to processes for forming a thin film on a substrate by decomposition of a thermally decomposable organometallic compound, and to thermally decomposable organometallic compounds and complexes which are useful in such processes. In another aspect, the present invention relates to a method of forming on a substrate a copper-containing superconducting oxide wherein at least one of the elemental constituents of the oxide is deposited by chemical vapor deposition from a metal organic source reagent. In a still further aspect, the present invention relates to a chemical vapor deposition reactor which may be usefully employed to form layers such as high temperature superconductor films on substrates.
2. Description of the Related Art
The recent discovery of the oxide-type high temperature superconductors (HTSC) has been followed by a tremendous effort to fabricate these materials in technologically useful forms. In particular, several thin film deposition methods have succeeded in producing films with short range epitaxy and critical current densities exceeding 10.sup.6 Amps/cm.sup.2. For the commercial potential of high temperature superconductivity to be realized, future process development must focus on cost-effective scale-up. Metal-organic chemical vapor deposition (MOCVD) is a particularly attractive method for forming these films because it is readily scaled up to production runs and because the electronics industry has a wide experience and equipment base in the use of CVD technology which can be applied to new MOCVD processes. Unfortunately, MOCVD processes require starting reagents ("source reagents") with sufficiently high volatility and chemical stability to permit gas-phase transport into the reactor chamber without premature decomposition. This requirement is difficult to meet for the Group IIA metals; in general, well-suited CVD source reagents for barium, calcium, strontium, and magnesium are not known.
The HTSC materials contemplated for fabrication by MOCVD include various types of materials, such as metal oxide superconductors comprising admixtures of metals from Groups IB, IIA, and IIIB of the Periodic Table. Illustrative materials of such type include the metal oxide superconductors of the yttrium-barium-copper type (YBa.sub.2 Cu.sub.3 O.sub.y) type, the so-called "123" HTSC materials, wherein y may be from about 6 to about 7.3, as well as materials where Y may be substituted by Nd, Sm, Eu, Gd, Dy, Ho, Yb, Lu, Y.sub.0.5 -Sc.sub.0.5, Y.sub.0.5 -La.sub.0.5, and Y.sub.0.5 -Lu.sub.0.5, and where Ba may be substituted by Sr-Ca, Ba-Sr, and Ba-Ca. Another illustrative class of superconductor materials includes those of the general formula (AO).sub.m M.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2, wherein the A cation can be thallium, lead, bismuth, or a mixture of these elements, m=1 or 2 (but is only 2 when A is bismuth), n is a number of from 1 to 5, the M cation is barium or stronium, and the substitution of calcium by strontium frequently is observed, as described in "High T.sub.c Oxide Superconductors," MRS Bulletin, January, 1989, pages 20-24, and "High T.sub.c Bismuth and Thallium Oxide Superconductors," Sleight, A. W., et al, MRS Bulletin, January, 1989, pages 45-48. Uses currently envisioned for these copper oxide superconductors include high speed opening switches, bolometers, and high frequency communications components such as mixers.
MOCVD of other compounds that contain barium, calcium, and/or strontium is subject to the same source reagent difficulty. Examples of other Group IIA-containing compounds that might desirably be employed in thin film form by MOCVD include BaTiO.sub.3, SrTiO.sub.3, BaF.sub.2, CaF.sub.2, and SrF.sub.2.
Barium titanate, BaTiO.sub.3, has been identified as a ferroelectric material with unique and potentially very useful properties. BaTiO.sub.3 in film or epitaxial layer form is useful in photonic applications such as optical switching, holographic memory storage, and sensors. In these applications, a BaTiO.sub.3 film is the active element. Applications in which the refractory materials may need to be deposited in film or layer form include integrated circuits, switches, radiation detectors, thin film capacitors, holographic storage media, and various other microelectronic devices.
The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are materials that are useful for scintillation detecting and coating of optical fibers. Thin films of the Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are potentially very useful as buffer layers for interfacing between silicon substrates and HTSC or GaAs overlayers. For example, a silicon substrate could be coated with an epitaxial layer of BaF.sub.2 /CaF.sub.2 whose composition is tailored for a close lattice match to the silicon. If the Ba/Ca ratio could be controlled precisely in the growing BaF.sub.2 /CaF.sub.2 layer, the lattice constant could be graded to approach the lattice constant of GaAs. Thus, a gallium arsenide epitaxial layer could be grown over the BaF.sub.2 /CaF.sub.2 interlayer, allowing the production of integrated GaAs devices on widely available, high quality silicon substrates. Another potential use of BaF.sub.2 /CaF.sub.2 interlayers would be as buffers between silicon substrates and polycrystalline HTSC films for applications such as non-equilibrium infrared detectors. Such an interlayer would permit the HTSC to be used in monolithic integrated circuits on silicon substrates.
Chemical vapor deposition (CVD) and metal-organic chemical vapor deposition (MOCVD) have been extensively described in the literature as techniques for depositing thin films of elements and compounds. A heat-decomposable compound (an organometallic in the case of MOCVD, referred to as the "source reagent") is contacted with a substrate which has been heated to a temperature above the decomposition temperature of the source reagent. The source reagent decomposes to deposit the element or metal on the substrate. By using more than one source reagent and adjusting the deposition parameters, deposition of compounds is possible. Control of key variables such as stoichiometry and film thickness and coating of a wide variety of substrate geometries are generally possible with MOCVD. Forming the thin films by MOCVD will permit the integration of these materials into existing device production technologies. MOCVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having similar crystal structures.
MOCVD requires that the element source reagents be sufficiently volatile to permit gas phase transport into the deposition reactor. The element source reagent must decompose in the reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds, especially HTSC materials, are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved only if the reagent can be delivered into the reactor in a controllable fashion. Close control of stoichiometry would also be desired, for example, in the application described above involving graded BaF.sub.2 /CaF.sub.2 interlayers.
In many cases, the source reagents are solids which can be sublimed for gas-phase transport into the reactor. However, the sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, so that it will be very difficult to control the stoichiometry of the deposited films.
Organo group II complexes are particularly problematic. Numerous Group II-containing acetylacetonates are volatile, including even some calcium and strontium derivatives. In fact, the barium complexes of 2,2,6,6-tetramethyl-3,5-heptanedione (Hthd) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (Hfod) have been shown to have limited volatility at elevated temperatures. Unfortunately, the compounds were reported to decompose at the temperatures required for sublimation (&gt;190.degree. C.) and were reported to be associated in the gas phase. Unpredictable gas phase association can lead to difficulties in controlling and reliably reproducing the desired gas phase transport of the source reagent into the reactor.
With the use of conventional bubblers in CVD operations, the bubbler is held at a temperature sufficiently high to sublime the reagent, and consequently significant and somewhat variable decomposition of the source reagents can occur during a single growth run. This premature decomposition causes variations in the composition as a function of thickness of the as-deposited film and poor reproducibility in film stoichiometry between different growth runs.
In exacting compositional control is particularly deleterious to high temperature superconducting thin films because the superconducting properties are extremely sensitive to the stoichiometry of the layer. Two approaches involving the use of nonconventional hardware have been tried to overcome this problem.
The first method eliminates the bubblers and uses a reactor tube which contains concentric tubes, each containing a boat filled with a single source reagent. A temperature gradient is applied along the tube to vaporize each material at the required temperature. There are several drawbacks to this method: (1) as with standard bubblers, significant decomposition occurs during given run because the reagents are held at high temperatures for the duration of the run, (2) temperature control is not as good as with standard bubblers, thus giving rise to wide variations in source reagent vapor pressure and consequently to wide variations in the stoichiometry of the as-deposited films, and (3) the boats need to be charged before each run, a step which is not consistent with a high volume commercial process.
The second method uses two bubblers in series. The first bubbler contains a volatile chelating ligand which presumably acts to stabilize and/or to lower the melting point of the source reagent which is contained in the second (downstream) bubbler. Stabilities on the order of a few hours have been realized with this method, which are sufficient for a single run. However, a fresh charge of source reagent is needed before each run. In addition, some enhancement of the vapor pressure of the source reagent occurs. Unfortunately, the amount of enhancement is not reproducible, which again causes variations in the stoichiometry of the as-deposited films.
In summary, the techniques heretofore employed for formation of Group IIA-containing thin films from relatively involatile reagents have not permitted efficient delivery of the reagents into the reactor or tight control of reagent ratios, with consequent adverse effect on achievement and reproduceability of the desired film stoichiometry.
It is an object of the present invention to provide an efficient method for forming high temperature superconducting material layers on substrates, and apparatus useful therefor.
It is an object of the present invention to provide novel source reagents useful for chemical vapor deposition of barium, calcium, strontium, and magnesium, and compounds containing one or more of these elements.
It is a further object of the present invention to provide a method for using the novel source reagents in MOCVD processes for HTSCS and other materials that incorporate barium, calcium, strontium, and/or magnesium.
It is another object of the invention to provide a method of making various Group II metal compounds based on beta-diketonates, which achieve significantly increased thermal stability of the product, as compared with corresponding compounds produced by prior art synthesis methods.
It is a still further object in the present invention to provide a chemical vapor deposition reactor having particular utility for formation of high temperature superconducting films having highly controllable stoichiometry.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.